An electric power supply, which uses a pulse width modulation (PWM) switching circuit as an electric power conversion circuit, and includes an LC filter interposed between the electric power conversion circuit and a load in order to eliminate noises as well as a feedback control system configured to make the output voltage fed to the load proportional to a command signal, is in use. At this time, a characteristic of the load encompasses a broad range from a capacitive load to an inductive load and substantially varies from zero to a maximum rating in magnitude. Therefore, such a robust PWM electric power supply is required that can meet both such an extensive variation in load and a variation in DC power supply voltage with a single controller.
In the PWM electric power supply like this, an amount of a feedback signal is preferably small for the sake of reducing an influence of noises on a controller and since a current detection sensor is generally expensive, a controller using only a voltage feedback network is desirably realized.
Then, a robust digital controller design method which can meet the requirements described above and is used in a PWM electric power supply has been proposed in a nonpatent document 1.
The digital feedback control system incurs a longer input dead time than does an analogue feedback control system. This input dead time is attributable mainly to a calculation time delay of a DSP, an analog-to-digital (AD) conversion time delay, a digital-to-analog (DA) conversion time delay, a delay in a triangular wave comparator, etc. The nonpatent document 1 focuses on these delays, expressing controlled objects (a PWM signal generator, a power conversion circuit and an LC filter) by a discrete-time system with two-orders higher than a continuous time system, in view of the input dead time and the conversion of a current feedback into a voltage feedback, and then proposing a configuration of a state feedback system which is intended to attain a given target characteristic with respect to the discrete-time system. Besides, in the nonpatent document 1, when after having equivalently converted the state feedback system into an output feedback system using only voltage, a robust compensator which is obtained by approximating this output feedback system is connected, whereby an approximately two-degree-of-freedom digital robust control system can be configured. Besides, by equivalently converting this robust digital control system, a digital integral controller using only the voltage feedback can be obtained.
In the nonpatent document 1, a method for configuring an approximately two-degree-of-freedom robust control system is shown which is intended to realize a first-order approximate model. In a robust digital controller incorporated with the control system like this, however, it has been difficult to curb a control input at the same time as increasing a degree of approximation. Therefore, it has been required to provide a design device of a robust digital controller whose magnitude of the control input does not need to be considered by anyone with a high degree of approximation maintained.
With respect to the two-degree-of-freedom robust digital control system proposed in the nonpatent document 1, a definite parameter determining means of the robust digital control system for increasing a degree of approximation is not shown. Therefore, a great deal of trial-and-error processes for determining the parameters was needed, thus requiring a lot of labor hours. Consequently, a definite parameter determining means by which anyone can easily design a robust digital controller has been needed to realize.
In order to solve the forgoing problems, a patent document 1 discloses a design device of a robust digital controller which has a high-degree of approximation and is incorporated with a new two-degree-of-freedom robust digital control system not required to consider magnitude of its control input.
FIG. 6 is a block diagram very roughly illustrating the robust digital control system disclosed in the patent document 1. In the figure, a model matching system with a disturbance qy and state feedback is shown. The disturbance qy mentioned here means a variation in output voltage due to an abrupt change in load or the like.
In FIG. 6, numeral symbol 101 denotes a DC-DC converter, which is controlled by an output voltage controller 100 to output an output voltage vo. In the present model matching system, this output voltage vo is shown as a result of having added, at an adding point 102, the disturbance qy to a voltage output from the DC-DC converter. Based on a given target value r, the output voltage controller 100 switching-controls switching elements, not shown, which make up the DC-DC converter 101 and so the output voltage vo is fed back in order that the output voltage vo becomes a desired voltage.
A transfer function Wqy from the disturbance qy to the output voltage vo in the model matching system is expressed by the following formula.
                                          W            qy                    ⁡                      (            z            )                          =                              Nm            ⁡                          (              z              )                                            Dm            ⁡                          (              z              )                                                          (                  Formula          ⁢                                          ⁢          5                )            where Nm(z) and Dm(z) denote a model matching system constant (a pole assignment is (H1 to H4)), and a function of a state feedback (f1 to f4) and constant of the DC-DC converter, respectively. The present model matching system is one for determining a response from the target value r to the output voltage vo.    Nonpatent document 1: “Robust PWM power amplifier using only voltage feedback to be operated by an approximately two-degree-of-freedom integral control” in the academic journal of Institute of Electronics, Information and Communication Engineers of Japan, Vol. J-85-C, No. 10, pp. 1 to 11, written by Koji Higuchi, Kazushi Nakano, Kuniya Araki, Fumie Kayano”    Nonpatent document 2: “A secondary model realizing robust PWM power amplifier operated by an approximately two-degree-of-freedom integral controller” in the academic journal of Institute of Electronics, Information and Communication Engineers of Japan, Vol. J-88-C, No. 9, pp. 724 to 736, written by Eiji Takegami, Koji Higuchi, Kazushi Nakano, Satoshi Tomioka, Kazushi Watanabe”    Patent document 1: Japanese Unexamined Patent Application Publication No. 2006-50723